1. Field of the Invention
The present invention relates to microwave couplers and, more particularly, to a triplate system directional coupler.
2. Description of the Related Art
There are many systems of microwave couplers. One example is a triplate system of directional couplers that are used in cellar phone stations.
A conventional triplate system directional coupler 1 is shown in FIGS. 5-8. The triplate system directional coupler 1 is composed of laminated plates. It comprises an intermediate dielectric board 2, first and second dielectric boards 3 and 4 for supporting the intermediate dielectric board 2 between them, first and second ground plates 5 and 6 provided on outer faces of the first and second dielectric boards 3 and 4, respectively, and a pair of first side ground plates 7 and 7xe2x80x2 and a pair of second side ground plates 8 and 8xe2x80x2 provided on side faces of the intermediate, first, and second dielectric boards 2, 3, and 4.
A pair of main tabs 10 are in contact with opposite ends of a main line (not shown) of copper foil that is provided on a face 9 of the intermediate dielectric board 2. A pair of auxiliary tabs 12 are in contact with opposite ends of an auxiliary line (not shown) of copper foil that is provided on the other face 11. Opposite ends 13-13xe2x80x2 and 14-14xe2x80x2 of the first and second side ground plates 7 and 7xe2x80x2 and 8 and 8xe2x80x2 are bent inwardly by 90 degrees and inserted between the first dielectric board 3 and the first ground plate 5, and the second dielectric board 4 and the second ground plate 6, respectively, for being joined together. Notches 15 are provided at positions corresponding to the main and auxiliary tabs 10 and 12 of the first side ground plates 7 and 7xe2x80x2.
To assemble the triplate system directional coupler 1, first of all, the intermediate dielectric board 2 is held between the first and second dielectric boards 3 and 4, and the opposite ends 13, 13xe2x80x2, 14, and 14xe2x80x2 of the first and second side ground plates 7, 7xe2x80x2, 8, and 8xe2x80x2 are engaged with the first and second dielectric boards 3 and 4, respectively. Then, the opposite ends 13, 13xe2x80x2, 14, and 14xe2x80x2, the intermediate dielectric board 2, and the first and second dielectric boards 3 and 4 are held between the first and second ground plates 5 and 6. Two grommets 16 and four rivets 17 are put through the opposite ends 13, 13xe2x80x2, 14, and 14xe2x80x2 of the first and second side ground plates 7, 7, 8, and 8xe2x80x2, the intermediate dielectric board 2, the first and second dielectric boards 3 and 4, the first and second ground plates 5 and 6, and their ends are deformed to join the respective components together.
In the above directional coupler, however, the respective components are joined together with the grommets 16 and the rivets 17 so that it is necessary to provide a space for the grommets 16 and rivets 17, resulting in the large plane area of the directional coupler. In addition, the first and second side ground plates 7, 7xe2x80x2, 8, and 8xe2x80x2 and the rivets 17 require much labor for assembling. If there is a gap between the respective ends 13, 13xe2x80x2, 14, and 14xe2x80x2 of the first and second side ground plates 7, 7xe2x80x2, 8, and 8xe2x80x2, the required ground and shield characteristics are not assured. For this reason, high precision work is required for the first and second side ground plates 7, 7xe2x80x2, 8, and 8xe2x80x2, requiring much labor.
Accordingly, it is an object of the invention to provide a compact directional coupler capable of simplifying the manufacture process and reducing the manufacturing cost.
According to the invention there is provided a directional coupler which comprises a plurality of dielectric boards; main and auxiliary lines provided between the dielectric boards; a ground plate provided on an outer face of one of the dielectric boards; and a conductive case provided over the dielectric boards so as to make contact with the ground plate.
It is preferred that the conductive case, the dielectric boards, and the ground plate are joined with grommets or rivets, or deformation of a part of the conductive case on the ground plate.